Device for measuring translation, rotation or velocity via light beam interference

ABSTRACT

The device for measuring translation, rotation or velocity includes at least a light source, a light detector, a first grating and a second grating, the first grating being mobile relative to the second grating. A incident beam reaches the first grating where it is diffracted in two beams whose directions are interchanged by the second grating, the resulting beams being then again diffracted by the first grating in an output diffraction direction where they interfere together. Both gratings are used in reflexion.

This application is a con of PCT/EP99/06057 Aug. 19, 1999.

The present invention concerns a device for measuring translation,rotation or velocity via interference of light beams diffracted bydiffraction gratings which are substantially parallel to each other.

European application 0 672 891 discloses a device for measuring relativedisplacements between a head unit and a scale. This device is of thetype where all diffraction gratings have the same spatial period orpitch P. The head unit has a light-emitting element (source), acylindrical lens to condense the light beam provided by the source and afirst diffraction grating used in transmission for splitting the lightbeam. The resulting diffracted beams fall onto a second grating arrangedon the scale where they are diffracted in reflexion. The head unitfurther comprises a third grating used in transmission for mixing thediffracted beams coming back from the scale and a light-receivingelement (photodetector). In all embodiments, the source and thephotodetector are spatially separated respectively from the first andthird gratings so that the head unit has relatively large dimensions.The distance between the mixing grating and the photodetector isactually needed because there is a plurality of interfering beams comingout of this mixing grating. Further, it is to be noted that for eachdiffraction event, at least one diffracted beam is not used. The unuseddiffracted beams represent a loss of light power, generate noise, andmay lead to spurious interferences. The efficiency of such a measuringdevice is thus relatively low.

U.S. Pat. No. 5,424,833 discloses a measuring device of another typewherein the first and third gratings are replaced by an unique indexgrating used in transmission with a pitch twice as large as the pitch ofthe scale grating. Thus, the scale grating, which is longer than theindex grating, has a pitch or spatial period smaller than that of thisindex grating. Further, all embodiments in this document are arranged tothat the incident beam falling on the index grating has a mainpropagating direction comprised in a plane perpendicular to the movingdirection of the scale grating and thus parallel to the lines of bothgratings. In order to spatially separate the light source and thephotodetector, this document proposes, in a first embodiment, to havesaid main propagating direction oblique relative to the directionperpendicular to the index grating in said perpendicular plane. In asecond embodiment, the incident beam falls perpendicularly onto theindex grating and a beam splitter is used which deflects theinterference beam coming back normally from the index grating into adirection different from the light source. The first embodiment needs anextended space in a direction perpendicular to the moving direction(measurement direction) and to the direction perpendicular to thegratings. The second embodiment has the following drawbacks: it needs anextended space between the source and the index grating, it is lessefficient, and it involves more parts.

European application 0 603 905 discloses a measuring device wherein twogratings are formed on the scale, a first one for splitting the lightbeam coming from the source and a second one with a pitch twice smallerfor interchanging the directions of the two used beams diffracted by thefirst grating. The mixing grating used in transmission is attached tothe photodetector. This arrangement is not very efficient because itsresolution is twice as small as the resolution of the device of U.S.Pat. No. 5,424,833 for gratings having pitches identical to those of thelatter. Further, the scale is transparent and either its two mainsurfaces are arranged for diffracting and/or reflecting light beams, oran additional mirror is needed. The scale is thus relatively difficultto manufacture.

An object of the invention is to provide an optical device for measuringrelative movements which has great measuring accuracy while remaining ofrelatively simple construction.

Another object of the invention is to provide such a measuring devicethe arrangement of whose various parts, in particular the scale orlonger grating, can be made within relatively large manufacturingtolerances without adversely affecting the accuracy of measurements.

Another object of the invention is to provide a measuring device of thistype wherein the variation in wavelength of the source and of itsangular spectrum have no influence on the accuracy of measurements.

Another object of the invention is to provide a device of this typeallowing a very flat arrangement which can easily be miniaturised.

A particular object of the invention is to provide a device of this typeat least partially integrated in a silicon or semiconductor substrate.

The invention therefore concerns a device for measuring translation,rotation or velocity via light diffraction including a light source, atleast one light detector, a first grating or first and fourth gratingsof the same spatial period and located substantially in a same firstplane, and a second grating or second and third gratings of the samespatial period and located substantially in a same second plane; thefirst and, where appropriate, fourth gratings being mobile along a givendirection of displacement relative to the second and, where appropriate,third gratings, this device being arranged so that a first light beamgenerated by said source defines a beam incident upon said first gratingwhere this incident beam is diffracted into at least a second beam and athird beam; so that these second and third beams then reach at leastpartially said second grating or, where appropriate, said second andthird gratings respectively, where they are respectively diffracted intoat least fourth and fifth beams whose propagating directions areinterchanged respectively with the propagating directions of said secondand third beams; so that these fourth and fifth beams then reach atleast partially said first grating or, where appropriate, said fourthgrating where they are respectively diffracted in a same outputdiffraction direction so that they interfere, said light detector beingarranged to detect at least partially light resulting from saidinterference; the first, second and, where appropriate, third and/orfourth gratings being used in reflexion.

The features of this measuring device allows an easy miniaturisation andits integration by microelectronic and microsystem technologies.

According to a preferred embodiment, said first and, where appropriate,fourth gratings belong to a portion of the device which is mobilerelative to said incident beam, said second and, where appropriate,third gratings being fixed relative to this incident beam.

According to a particular embodiment, the first and, where appropriate,fourth gratings have a pitch or spatial period which is twice as largeas that of the second and, where appropriate, third gratings, saidsecond and third beams being diffracted respectively into the <<+1>> and<<−1>> orders, said fourth and fifth beams being diffracted respectivelyinto the <<−1>> and <<+1>> orders and these fourth and fifth beams beingrespectively diffracted into the <<+1>> and <<−1>> orders in said sameoutput diffraction direction by said first or, where appropriate, fourthgrating.

According to a preferred feature of the measuring device according tothe invention, the light from said incident beam forming said second,third, fourth and fifth beams and finally detected by the detectorreaches said first grating at an angle of incidence which is not zero ina plane perpendicular to lines forming the gratings, this angle ofincidence being sufficient so that the light source providing said lightand the detection region of the detector receiving said light arespatially separated from each other in projection in a planeperpendicular to said lines.

According to a particular feature, said output diffraction directiondefines an angle, in said plane perpendicular to lines forming thegratings, which has a value substantially equal to the angle ofincidence of the incident beam multiplied by <<−1>> relatively to anaxis perpendicular to said gratings, only light interfering along thisoutput diffraction direction being used for measuring a displacement.Thus, the optical arrangement is fully symmetrical and so reciprocal.

Other objects, particular features and advantages of the presentinvention will appear more clearly upon reading the following detaileddescription, made with reference to the annexed drawings, which aregiven by way of non-limiting example, in which:

FIG. 1 shows schematically an optical device for measuring a relativedisplacement,

FIGS. 2 and 3 show schematically a first embodiment of a measuringdevice, according to the invention;

FIG. 4 shows schematically the spatial distribution of the light beamsused for the displacement measurement of a second embodiment;

FIGS. 5 and 6 show schematically a third embodiment of a measuringdevice, according to the invention;

FIGS. 7, 8 and 9 show schematically three other embodiments of theinvention;

FIG. 10 shows schematically and partially an embodiment allowing anabsolute measurement of the relative position between a mobile scale andthe fixed portion of the displacement measuring device;

FIGS. 11 to 15 show schematically various alternatives for defining areference position of the mobile scale of the displacement measuringdevice;

FIGS. 16 and 17 show schematically two other embodiments of theinvention;

FIGS. 18 and 19 show schematically an embodiment allowing measurement ofdisplacement along two orthogonal directions;

FIG. 20 shows schematically another embodiment of the invention in whichthe beam emitted by the light source and the interfered beam propagateparallel to the measured displacement direction.

FIG. 1 shows a translation measuring device including a light source 2which supplies a first beam FI, which reaches a first transparentstructure 4 on one surface of which is arranged a first grating 6 ofperiod Λ. Beam FI is diffracted into the <<+1>> and <<−1>> orders andgenerates two beams 8 and 10. Beams 8 and 10 reach respectively secondand third gratings 12 and 14 where they are reflected and diffractedrespectively into the <<−1>> and <<+1>> orders. Beams 16 and 18resulting from these two diffractions propagate symmetrically to beams10 and 8 and are joined together as they reach a fourth grating 20 wherethey are diffracted, respectively into the <<+1>> and <<−1>> orders,along a same first direction of diffraction offset angularly by angle αrelative to an axis perpendicular to grating 20, this angle α beingidentical in absolute value to angle of incidence α of beam FI incidentupon first grating 6.

The two beams generated by the diffraction of beams 16 and 18 in grating20, along the aforementioned first direction, interfere and togetherform a beam FR which again passes through transparent structure 4 and isthen directed towards light detector 22 arranged for measuring thevariation in the luminous intensity of beam FR resulting from saidinterference. The first and fourth gratings are situated in a same firstgeneral plane and arranged on a same face of transparent structure 4.Likewise, second and third gratings 12 and 14 are arranged in a samesecond general plane of the device. Grating 14 is arranged at thesurface of a reflective support 24 which is fixed relative to structure4, while grating 12 is arranged at a surface of a mobile reflectivesupport 26 moving along a direction X parallel to the aforementionedfirst and second general planes. In this embodiment, mobile portion 28,formed of support 26 and grating 12 remains in a fixed position alongaxis Z during measured displacements.

The path travelled by beams 8 and 16, on the one hand, and beams 10 and18 on the other hand, are identical. Consequently, the phase shiftbetween the two beams 16 and 18 incident upon grating 20 depends solelyupon the displacement of mobile portion 28. Those skilled in the artknow how to calculate the phase shift generated by a displacement alongaxis X of this mobile portion 28 for beam 16 generated by thediffraction of beam 8 in grating 12, this phase shift increasingproportionally with the displacement of moving portion 28 and theluminous intensity of beam FR detected by detector 22 varyingperiodically. Measurement of this periodic variation in the luminousintensity of beam FR allows the displacement of mobile portion 28 to bedetermined with great accuracy.

Gratings 6 and 20 have a spatial period Λ and gratings 12 and 14 have aperiod which is substantially two times smaller, i.e. substantiallyequal to Λ/2 and preferably equal to Λ/2. This ratio between the spatialperiods of the different gratings allows two reciprocal optical paths tobe obtained defining a symmetry relative to axis Z. Indeed, due to theparticular arrangement of the aforementioned different spatial periodsan incident beam FI at point A of grating 6 generates two diffractedbeams 8 and 10 which are diffracted respectively at points B1 and B2along two directions which are symmetrical to the directions of beams 8and 10 relative to axis Z. Consequently, beams 16 and 18 meet at point Csituated on gratings 20. There is thus perfect superposition of the twobeams interfering along said first direction of diffraction.

It will be noted however that the four gratings can be situated indifferent general planes if required as long as the relativedisplacements are effected in displacement planes parallel to thesegeneral planes. However, such an arrangement loses certain of theadvantages of the device of FIG. 1, in particular its independencerelative to the wavelength λ of beam FI and its angle of incidence α.That is why, although such a solution is not excluded, an arrangement inaccordance with FIG. 1 is preferred. Those skilled in the art candemonstrate mathematically that the intensity of beam FR resulting fromthe interference is independent of angle α and the wavelength of beam FIwhen gratings 6 and 20 are situated in a first general plane andgratings 12 and 14 are situated in a second general plane of the device.This feature is particularly advantageous for light sources emittingwith a certain divergence or numerical aperture in a spectral band of acertain width, i.e. non monochromatic.

According to a particular feature of the present invention, beam FIincident upon first grating 6 has an angle of incidence α which is notzero. Consequently, in the plane of FIG. 1 which is parallel to thedirection of displacement of mobile portion 28 and perpendicular tolines 30, 31, 32 and 33 of gratings 6, 20, 12 and 14, the point ofincidence A on grating 6 and the point of interference C on grating 20are separated spatially so that source 2 and detector 22 are separatedspatially in projection in this plane and can thus be arranged so as tobe globally aligned along a direction parallel to direction ofdisplacement X. This allows very flat measuring devices to be obtainedgiven that the source and the detector can both be arranged in a planeparallel to the measured displacement direction.

Another consequence of non-zero incidence angle α is to prevent thespurious z-dependent modulation signal due to self-mixing when thesource is a semiconductor laser.

The device according to FIG. 1 is favourable for measuring a relativedisplacement between two bodies situated in a same general plane.

Given that only diffraction orders <<+1>> and <<−1>> of grating 6 areuseful, this grating 6 is arranged so that the majority of the luminousintensity of beam FI is diffracted into these two diffraction orders toform respectively beams 8 and 10. In particular, the light emitted intodiffraction order <<0>> is minimised. Likewise, in the event that thesecond diffraction order may intervene, grating 6 is arranged so thatthe light diffracted into this second order is relatively weak.

By way of example, for a wavelength Λ=0.67 μm and an angle of incidenceα=10°, diffraction grating 6 is formed in dielectric layer 36 ofrefractive index approximately n=2.2, in particular made of Ta₂O₅ orTiO₂ deposited by a technique known to those skilled in the art, onglass substrate 4, the total thickness E₁ of this layer being comprisedbetween 0.4 and 0.5 μm. The depth P₁ of the grooves situated betweenlines 30 of grating 6 is comprised between 0.30 and 0.35 μm.Transmission of approximately 80% of the total luminous energy of beamsFI is thus obtained in diffracted beams 8 and 10. Defining the grating 6in layer 36 composed of a high index dielectric material is particularlyadvantageous since it allows a large diffraction efficiency of the<<+1>> and <<−1>> orders to be obtained with a shallower groove depth P₁than in a lower index layer, or than directly in the transparentstructure 4.

Those skilled in the art can also optimise the profile of the section ofgrating 6 along the transverse plane of FIG. 1 to further increase thisselective transmission of the luminous energy or define other gratingprofilers in layers of different transparent materials such as SiO₂ orpolymers or solgels. It will be noted that, given that the diffractionevents at point C form a reciprocal situation with the diffractions atpoint A, a difference in the percentage transmitted into the <<+1>> and<<−1>> orders at point A is re-established during diffraction at point Cat angle α so that the contributions of beams 16 and 18 along thedirection of diffraction selected are identical, which leads to maximumcontrast for the interference. It will also be noted that thediffraction efficiency in the aforementioned example is substantiallyindependent of the polarisation of the incident light. The lightdiffracted into <<0>> order is practically zero. With a period Λ=1 μm,diffraction orders greater than 1 do not exist.

Those skilled in the art will choose for reflection gratings 12 and 14 acorrugated metal surface. It is known that such metal gratings exhibithigh diffraction efficiency for beams 8 and 10 of TM polarization only.High diffraction efficiency for the TE polarization requires a largegroove depth which is very difficult to obtain in practice when theperiod is of the order of the wavelength. Furthermore, it is practicallyvery difficult to obtain such metal grating exhibiting comparable largediffraction efficiency for both TE and TM polarizations of beams 8 and10 as is requested in case the light source is unpolarized. An object ofthe invention is to provide high diffraction efficiency for the TEpolarization, and for both TE and TM polarization, by using a gratingstructure comprising a flat mirror substrate 26 or 24, a dielectriclayer 38 and 40, the grating 12 or 14 being realized in the dielectriclayer 38 or 40. Such structure associates the diffraction of grating 12or 14 with the reflection of the reflective substrate 26 or 24 in orderto give rise to constructive interference effects in the direction ofbeam 16 or 18.

In a particular example, gratings 12 and 14 are both formed of adielectric layer respectively 38, 40 also having a refractive indexn=2.2. With a total thickness E₂=0.34 μm and a depth P₂=0.18 μm for thegrooves situated between lines 32 and 33, the luminous intensitydiffracted into the <<−1>> order for grating 12 and the <<+1>> order forgrating 14 is approximately 50%, the remainder being essentiallydiffracted into the <<0 >> order. Given that beam 8 is diffracted to theright of the direction perpendicular to grating 6, the light diffractedinto the <<0>> order by grating 12 does not disturb the measurement inany way since it is not received by detector 22. Likewise, the lightdiffracted at B2 into the <<0>> order reaches grating 20 at a distancefrom point C comparable to the distance separating point C from point A.It is thus easy to arrange detector 22 so that the light diffracted atpoint B2 into the <<0>> order is not detected. This fact favours inparticular a ratio between wavelength λ and period Λ generatingpropagation of beams 8 and 10 to the right and left of the directionperpendicular to grating 6 respectively.

The arrangement of gratings 12 and 14 described in the examplehereinbefore is provided for a situation in which the light received isnot polarized. However, if the light is TE polarised (electric fieldvector parallel to the grating lines), thickness E₂ of gratings 12 and14 is approximately 0.1 μm, while the depth P₂ is situated at around0.08 μm and can even be equal to thickness E₂. Substrates 24 and 26 aremade for example of aluminum or coated with an aluminium film or anothersuitable metal. Under these conditions, approximately 80% of theluminous intensity of beams 8 and 10 is dffracted respectively in beams16 and 18. For a TM polarisation (electric field vector perpendicular tothe grating lines), one can omit the dielectric layer and the aluminiumsubstrate is micro-machined with a groove depth of approximately 0.12μm. In a variant, substrate of any type is micro-machined, then coatedwith a metal film. Thus, the luminous intensity diffracted in beams 16and 18 is approximately 70%. Again, the profiles of gratings 12 and 14in the plane of FIG. 1 can be optimised by those skilled in the art soas to increase the transmission of luminous energy in the respectiveuseful directions, in proportions substantially equal but notnecessarily equal at points B1 and B2. Other layer materials like otheroxides, fluorides, polymers, solgels can be chosen and deposited orcoated by different techniques like vacuum deposition, spinning,dipping, in which the grating can be achieve by dry or wet etching,lift-off, photo inscription or moulding techniques.

Dielectric layer 42 of grating 20 has a thickness E₁ and a groove depthP₁ substantially identical to those of grating 6 so as to assurereciprocity of the diffraction even at C relative to the diffractiveevent at A. The diffraction efficiencies at C correspond to those givenhereinbefore for the diffractions occurring at A.

Finally, in a variant, transparent structure 4 is in two portions whichare mobile in relation to each other and carry respectively the firstand fourth gratings 6 and 20, while the second and third gratings 12 and14 are both attached to one of these two portions.

FIGS. 2 and 3 show a first embodiment of the invention. Beam FIgenerated by a source which is not shown passes through transparentstructure 44 and reaches grating 46, at an angle of incidence α, whereit is diffracted into the <<+1>> and <<−1>> orders to form beams 8 and10, as in the first embodiment. However, this second embodiment differsfrom the first in that beam 8 is diffracted to the left of the directionperpendicular to grating 46. By way of example, the light wavelengthλ=0.6 7 μm, angle of incidence α=20° and period Λ=2 μm.

Beams 8 and 10 reach grating 48 arranged at the surface of reflectivesubstrate 50. Beams 8 and 10 are respectively diffracted by grating 48into diffraction orders <<−1>> and <<+1>> to form respectively beams 16and 18 which are joined as they reach again grating 46 where they arediffracted along a same diffraction direction, at an angle α relative tothe direction perpendicular to grating 46. Beam FR resulting from thisinterference again passes through transparent structure 4 prior to beingdetected at least partially by a detector which is not shown.

It will be noted that substrate 50 is here staitonary relative to thesource and the detector, while structure 44 is mobile along direction X.The luminous intensity of beam FR varies periodically as a function ofthe displacement of structure 44 relative to substrate 50. This detectedluminous intensity and the periodic variation therein allows therelative displacement between structure 44 and substrate 50 to beaccurately determined.

In order to optimise the transmission of the luminous energy of beam FIin diffracted beams 8 and 10 and also in order to optimise thetransmission of the luminous energy of these beams 16 and 18 in beam FR,for α,λ and Λ given hereinbefore, grating 46 is formed of a dielectriclayer 52 of refractive index n=2.2 approximately and having a thicknessE₁ comprised between 0.35 and 0.40 μm with a groove depth P₁ equal toapproximately 0.24 μm. It will be noted that this grating structure andthese values are given by way of non-limiting example and have beendetermined for a transparent structure 44 with an index of approximatelyn=1.5. Under these conditions, approximately 60% of the luminous energyof beam FI is transmitted in diffracted beams 8 and 10 in substantiallyequal parts, independently of the polarisation of the light. Theluminous intensity transmitted into the <<0>> order is low. It isapproximately zero for TE polarisation while it reaches approximately 5%for TM polarisation.

In the event that the light is not polarised, second grating 48 isformed by a dielectric layer 54 of refractive index n=2.2 having a totalthickness E₂ comprised between 0.25 and 0.30 μm with a groove depthP₂=0.22 μm. As in FIG. 1, a high efficiency grating comprising adielectric layer 54 and a reflective substrate 50 is provided, thegrating 48 being made in said dielectric layer. Approximately 55% of theluminous intensity of beams 8 and 10 is diffracted respectively in beams16 and 18. Preferably, the refractive index of the dielectric layersmentioned is greater than 1.8. For the sole TE polarised light, theluminous intensity diffracted into the useful orders at grating 48 canbe increased to approximately 70% with a thickness E₂ slightly greaterthan 0.30 μm. Under these conditions, it is possible to obtain 70% ofthe energy transmitted in beams 16 and 18 while the luminous energydiffracted into the <<0>> order is very low; which is not the case forTE polarisation when thickness E₂ is less than 0.30 μm.

The numerical example given here thus allows the luminous energytransmitted into diffraction order <<0>> in grating 46 to be reduced tothe maximum and also, although to a lesser extent, in grating 48. Then,the light transmitted into the second diffraction order to relativelysmall. Consequently, the only significant interference is that generatedby the diffraction of beams 16 and 18 in grating 46 respectively intothe <<+1>> and <<−1>> orders, at angle of diffraction α. This favourablesituation results essentially from the fact that the transmission ofbeams 16 and 18 into the <<0>> order of diffraction and the ordersgreater than the first order of diffraction at point C is relativelylow, or even zero. Thus, a detector situated in proximity to point Cessentially receives beam FR as a light signal varying alternately as afunction of the displacement of substrate 44. The other contributionsreceived by this detector generate a substantially constant signalindependent of the relative displacement between substrate 50 andstructure 44.

In the example given here, the light is essentially transmitted in theuseful orders and the low intensity of the light transmitted into the<<0>> order of diffraction at points A and B1 allow any light generatinga constant signal to be reduced to the maximum for the luminousintensity received by the detector. It will also be noted that giventhat the diffraction at point C into the <<0>> order is relatively low,any interference with a diffraction into the second order can generateonly a small luminous variation and thus a minor disturbance for themeasurement signal propagating at angle α and formed by beam FR. In theexamples given hereinbefore, most of the luminous intensity of beams 16and 18 is diffracted respectively into the <<+1>> and <<−1>> orders, theamplitudes of the diffracted beams into other orders being small orzero. It is to be noted that no particular care must be taken of theluminous intensity in the zero and second orders when the light sourceis broadband source like a Light Emitting Diode (LED) since theircontribution in the detected signal only amounts to a DC componentbecause of the short coherence length of a LED.

In order to be able to determine the direction of relative displacementbetween structure 44 and substrate 50, grating 48 has been divided intotwo regions R1 and R2 along the direction perpendicular to direction ofdisplacement X (FIG. 3). In region R2, grating 48 is also divided intotwo distinct regions R3 and R4. In region R3, lines 58 of grating 48 arein phase over the two regions R1 and R2. However, in region R4, lines 58have a discontinuity given that the part of these lines situated inregion R2 is offset by Λ/8 relative to the part of these lines situatedin region R1. Grating 48 is arranged relative to the light source sothat beam 8 reaches grating 48 in region R3 while beam 10 reaches intoregion R4. In these conditions those skilled in the art can calculatethat the offset of Λ/8 provided in region R4 finally generates a phaseshift of II/4 between beams 16 and 18 incident upon grating 46 at pointC. Consequently, the luminous intensity resulting from the interferenceoriginating from region R1 has a phase shift of II/2 relative to theinterference originating from region R2. By separately detecting thecontributions from region R1 and R2, the detector receives twoalternating luminous intensity signals phase shifted by II/2 in relationto each other. In a variant, it is possible to provide three gratings inparallel with an offset of Λ/6 to give three luminous intensity signalsphase shifted by 120°. If beams 8 and 10 are not spatially separatedwhen they reach grating 48, region R2 does not have to be separated intoregions R3 and R4. Region R2 as a whole is offset by Λ/16 with respectto region R1 in order to provide an optical intensity phaseshift ofII/2, or by Λ/12 for a phaseshift of 120°. Grating 48 can also bedivided into four regions similar to R1 and R2 with three regions havingrespectively offsets of Λ/16, Λ/18, 3Λ/16 relative to the last one inorder to obtain the full set of four quadrature optical power signals.

Thus, on the basis of these two, or three or four separately detectedsignals, the electronic system of the measuring device can determine thedirection of relative displacement between structure 44 and substrate 50and interpolate finely within the electric period Λ/4 of the luminousintensity resulting from said interference to further increase theaccuracy of the measurement. It will be noted that, in the case of thedevice of FIG. 1, this electric period is Λ/2.

It will be noted that a variation in the spacing between this structure44 and substrate 50, i.e. a variation in the distance separatinggratings 46 and 48 has no influence on the measurement of thedisplacement along axis X, the two optical paths between points A and Cremaining identical and the phase shift between the two contributionsforming beam FR and originating respectively from beams 16 and 18remaining dependent solely on the relative displacement along axis X.

Finally, it will be noted that the phase shift for a given displacementis twice as large in this second embodiment than in the first embodimentof FIG. 1.

FIG. 4 shows schematically a second embodiment in which transparentstructure 44 is stationary relative to source 2 and detector 22,reflective substrate 50 being mobile. Gratings 46 and 48 are the same asthose described with reference to FIG. 2. FIG. 4 is given to allow thelight useful for the displacement measurement provided by source 2 to bevisualised. This source 2 generates a beam FI which has a divergence ornumerical aperture and which reaches grating 46 at an angle of incidencevarying continously within a range of given values. It will be notedthat this range of values can include the value α=0, i.e. an incidenceperpendicular to grating 46. This beam FI generates beams 8, 10, 16, 18and FR as described hereinbefore. The numerical aperture of beam FIgenerates a divergence of these diffraction beams.

Since detector 22 is arranged relative to source 2 so that theirprojections in a plane perpendicular to the lines of gratings 46 and 48are not superposed, although they are globally aligned along asubstantially parallel direction to the direction of displacement, onlythe light which is comprised in a partial beam FI* and illuminatesregion RA of grating 46 (comprised between the two arrows in thedrawing) forms the partial beam useful for the displacement measurement.According to the invention, the totality of light FI* incident uponregion RA has an angle of incidence which is not zero, but sufficientlylarge for the light finally incident upon detection element 80 to bespatially separated from the light forming beam FI*, in projection in aplane perpendicular to the lines of gratings 46 and 48 corresponding tothe plane of the drawing of FIG. 4. When detection element 80 issituated in direct proximity to region RC where partial beams 16* and18* arrive which generate partial beam FI* detected by detector 22, thiscondition corresponds to a spatial separation of regions RA and RC ofgrating 46. Beam FI* which is useful for the displacement measurementthus generates partial beams 8* and 10*, which reach grating 48respectively in regions RB1 and RB2. From there they are diffracted toform partial beams 16* and 18* and are joined in region RC of grating 46where they are diffracted along a same direction to form partial beamFR* of beam FR.

In conclusion, whatever the divergence or numerical aperture of beam FI,only partial beam FI* contributes to the displacement measurement andonly regions RI, FB1, RB2 and RC define the active regions of gratings46 and 48 in which the optimising conditions for maximum diffractionefficiency and maximum contrast of the detected interference signal mustbe fulfilled. It will also be noted that the light forming beam FI* canhave a wide spectrum.

Hereinafter, the numerical references already described will not bedescribed again in detail, since they were only given as an example. Itis indeed an object of the invention that the gratings can bemanufactured with large tolerances without affecting the measurementaccuracy.

With reference to FIGS. 5 and 6 a third embodiment of the invention willbe described hereinafter, wherein an angular displacement of a wheel 60is measured, said wheel having at its periphery a grating 62 formed oflines 64 parallel to the axis of rotation of wheel 60. Grating 62defines a scale of period Λ. Facing grating 62 there is provided ameasuring head 66 formed of a transparent structure 68 having on itsface opposite grating 62 a diffraction grating 70 having a period Λ/2.The ratio of the period of grating 70 to the period of grating 62 issubstantially 1/2 when the angle between the normals to grating 62 atpoints A et C is close to zero. This ratio is smaller than 1/2 when theradius of wheel 60 is small and when the spacing between gratings islarge. On the other face of structure 68 are arranged a light source 72and a decoder 74. Beam FI generated by source 72 passes throughstructure 68 and reaches grating 62 where it is diffracted in reflectionessentially into the two orders of diffraction <<+1>> and <<−1>>. BeamFR, resulting from the interference of beams 16 and 18 diffracted inreflection at angle α at point C, again passes through structure 68prior to being detected by detector 74. Grating 70 is formed in areflective substrate 76 deposited at the surface of transparentstructure 68.

An incremental angle of rotation of wheel 60 corresponds to period Λ ofgrating 62. Thus, for every displacement of grating 62 relative tomeasuring head 66 there is a corresponding angle at centre of wheel 60.Consequently, the processing of the alternating luminous signal detectedby detector 74 allows an angle of rotation of wheel 60 to be accuratelydetermined.

As in the second embodiment, the direction of rotation of wheel 60 canbe detected. In order to do this, grating 70 shown in plane in FIG. 6has two regions R1 and R2 in which the lines 78 of grating 70 are offsetby Λ/16. This offset provided at points B1 and B2 finally generates anoptical intensity phase shift of II/2 in beam FR between the twocontributions originating from regions R1 and R2.

FIG. 7 shows a fourth miniaturised embodiment which is partiallyintegrated in a semiconductor substrate 82. This substrate 82 has anaperture 84 wherein is arranged a collimation ball for the light emittedby electroluminescent diode 88 arranged at or close to the surface ofball 86. Diode 88 is arranged so that the central axis of beam FIleaving ball 86 has an angle of incidence which is not zero whenreaching grating 90 of period Λ. On the face of substrate 82 situatedfacing grating 90 arranged on reflective substrate 112 there is provideda reflection grating 92 of period Λ/2. This grating 92 can be eithermicro-machined directly in substrate 82, in particular in silicon, or beobtained by deposition of one or more layers by deposition techniquesknown to those skilled in the art. In particular, it is possible todeposit a metal layer followed by a dielectric layer. The lines ofgrating 92 can be obtained either by micro-machining the dielectriclayer or by a two phase deposition, the deposition effected in thesecond phase forming the lines of grating 92. The resulting beam FRoriginating from diffraction of beams 16 and 18 in grating 90 is finallydetected by detector 98 integrated in substrate 82. Such detectors areknown to those skilled in the art, as is the electronic circuit used forprocessing the light signals received by said detector 98.

It will be noted that the light detector can be formed by a unit whichis materially distinct from substrate 82, in particular by a detectionunit preceded by a focusing element. In such case, this detectionassembly is arranged either in another aperture, or in a recess providedon the face of this structure 82 situated opposite grating 90.

FIG. 8 shows a fifth miniaturised and partially integrated embodiment.Semiconductor substrate 82 comprising integrated detector 98 has arecess 100 in which is arranged the source formed of electroluminescentdiode 88 and transparent ball 86. The bottom of recess 100 is closed bya transparent layer 102, in particular made of SiO₂ or Si₃N₄, arrangedon one face of substrate 82 on the side of detector 98. At the surfaceof this layer 102 is arranged a dielectric layer defining grating 104 ofperiod Λ. Facing grating 104 is arranged reflection grating 106 ofperiod Λ/2 at the surface of a mobile reflective scale 108.

FIG. 9 shows a sixth entirely integrated embodiment. The displacementmeasuring head is formed by semiconductor substrate 82 in which areintegrated detector 98 and light source 110. Preferably, source 110 isdirectly integrated in substrate 82. In a variant, source 110 can bemanufactured separately and arranged at the surface of substrate 82 orin a recess provided for the source. Although source 110 emits with alarge numerical aperture in several directions, only a portion of thebeam generated defines beam FI diffracted by gratings 90 and 92 isfinally detected by integrated detector 98. The optical paths of the twoend beams FIA and FIB of partial beam FI have been shown so as tovisualise the spatial distribution of the different diffracted beamsuseful for the relative displacement measurement between substrates 82and 112. The two end rays of each beam are referenced respectively bythe lens <<A>> and <<B after the previously used numerical reference.This sixth embodiment allows an ultimate miniaturisation of themeasuring device according to the invention and the integration thereofin mechanical and micromechanical devices.

FIG. 10 shows schematically a seventh embodiment of the invention whichdiffers from the sixth in that, in place of a single grating 90, threegratings 90A, 90B and 90C are provided, arranged next to each other andhaving respectively three different, although relatively close, spatialperiods Λ1, Λ2 and Λ3. Grating 92 is also replaced by three gratings(not shown) situated facing the three gratings 90A, 90B and 90C, andeach having a spatial period which is two times smaller than the spatialperiod of the grating which it faces. For each of the pairs of gratings,the application of the optical principle disclosed in the presentinvention is identical. By selecting appropriate values of Λ1, Λ2 andΛ3, the light intensities, received by a detector having three distinctdetectio zones for the three pairs of gratings, define a signalcorresponding to a single relative position between substrate 82 andsubstrate 112. Such a device thus enables the absolute position of themobile portion to be defined relative to the fixed position of thedevice. This constitutes an application of the Vernier principle. Thedevice can contain N paths of different periods to assure univocalcoding of each measured relative position between substrates 82 and 112.

FIGS. 11 to 14 shows schematically four alternative embodiments of themobile portion relative to the light source and the detector each ableto be arranged in any of the embodiments described hereinbefore todefine at least one reference position between the fixed portion and themobile portion of the displacement measuring device.

According to the variant of FIG. 11, in addition to base grating 116 ofconstant period Λ or Λ/2, there is provided beside this latter anothergrating 118 of variable spatial period and decreasing to substantiallyan identical period to that of grating 116, able to perform identicallyto grating 116 on a certain number of lines, to increase again. Thereference position REF is defined by the symmetrical axis of grating118. The variant of FIG. 12 differs from that of FIG. 11 in that agrating 120 is provided beside grating 116 whose period varies byincreasing or decreasing passing from a value higher than the value ofthe period of grating 116 to a lower value than the latter. Referenceposition REF corresponds to the middle position of the place ofcoincidence between the periods of gratings 116 and 120 able to extendover a certain number of lines.

When the light beam sweeps gratings 118 of FIG. 11 or grating 120 ofFIG. 12, an interference signal is generated on passing across thereference region allowing the displacement measuring detector or anotherdetector to determine reference position REF. This originates from thefact that grating 118 or 120 has only in the reference region a periodhaving a ratio 1/2 or 2/1 with the grating situated opposite on thefixed portion of the displacement measuring device. In other words,there is coding of an absolute or reference position by mutual spatialcoherence of the two gratings.

FIG. 13 shows another variant wherein there is provided beside grating116 a grating 122 of decreasing the increasing variable period passingfrom a period higher than that of grating 116 to a lower period. Grating122 has symmetry relative to reference position REF situated between twointerference signals occurring at two reference positions REF1 and REF2where the period is identical to that of grating 116. Grating 122 thusallows two reference positions REF1 and REF2 to be determined, whichallows the detected signal processing means to define with greataccuracy the central reference position REF.

In FIGS. 11 to 13 it will be noted that in the event that grating 116has a period Λ/2, the mutual coherence at the reference location must beverified at least partially for the diffraction events at thediffraction points or regions of incident beams 8 and 10. Consequently,the variant of FIG. 13 can only define one reference position with aspacing between these two points or regions substantially equal to thedistance between REF1 and REF2.

FIG. 14 shows another alternative embodiment wherein the mobile portionrelative to the light source includes grating 126 of period Λ/2. Asecond grating 128 is provided beside grating 126, these two gratings126 and 128 being arranged facing the grating of constant period Λ.Grating 128 is formed of lines 130 defining a period Λ/2 with twodiscontinuities defining an phase shift of offsetting of one portion ofgrating 128 relative to the corresponding lines 132 of grating 126.Grating 128 thus has a first offset of Λ/4 increasing a space betweentwo lines 130 to 3Λ/4. At a certain distance from this offset a secondoffset of Λ/4 is provided decreasing from period Λ/2, generating a spaceΛ/4 between two other lines 130.

FIG. 15 shows the variation in the luminous intensity detected by adetector as a function of the displacement of grating 128 when the lightbeam passes through the region including the two offsets of oppositedirections described hereinbefore. First, the component AC of theintensity I of beam FR defined hereinbefore decreases given that oneincreasing portion of this beam includes an interference product havinga phase difference of II. When more than half of the first offset ofgrating 128 has been passed through or the second phase jump is reached,the component AC of intensity I again increases to the maximum beforeagain decreasing and then increasing towards the initial mean value.Graph 134 of FIG. 15 thus defines three reference points F1, F2 and F3allowing three reference positions to be defined or, using a processingunit, central reference position F2 to be accurately defined. It will benoted here that it is possible in another variant to provide a singlephase jump of Λ/4 thus generating a single minimum in the AC componentof intensity 1.

FIG. 16 shows another embodiment of the invention which is particularlyadvantageous and able to be miniaturised. The device includes on the onehand a substrate 82 on one face of which is arranged a light source, inparticular an electroluminescent diode or a light source integrated in asemiconductor region of substrate 82 and known to those skilled in theart. As in the embodiment of FIG. 9, this source 110 can be a poroussilicon light emitting zone, an electroluminescent polymeric emitter ina recess zone, or a LED chip bonded onto substrate 82. This embodimentdiffers essentially from the sixth embodiment in that a partial beampropagating to the right of light source 110 and another partial beampropagating to the left of said source are used for the displacementmeasurement. Thus, to the left and right of source 110 are provided twogratings 92 and 92′ of period Λ/2. On either side of these tworeflection gratings are arranged two light detectors 98 and 98′integrated in regions of semiconductor substrate 82. The optical pathsof the beams diffracted to the left and right of source 110 and the twopartial beams used for the displacement measurement are substantiallysymmetrical. Facing the face of substrate 82 having gratings 92 and 92′is arranged a grating 90 of period Λ on a reflective substrate 112.

In order to determine the direction of displacement of grating 90 and tointerpolate in a period of the detected luminous intensity signal, avariant provides an offset of Λ(m/4+1/16) between gratings 92 and 92′where m is an integer number.

Consequently, the alternating signal detected by detector 98 is phaseshifted by II/2 relative to the alternating signal detected by detector96′. However, in order to be free of any dilatation problem, it ispreferable to provide two additional gratings phase shifted or offset byΛ/16 on each side of source 110. The mention of possible expansion leadsus to mention here an application of the device according to theinvention to temperature measurements by expansion of the substrateformed of materials determined for such application. This is importantin rotating or translating mechanical systems where the temperature ofthe moving parts has to be monitored as a criterion for the system'ssafety or lifetime.

FIG. 17 shows another particularly advantageous embodiment which differsto that described in FIG. 16 in that an opening 100 is provided in thesilicon substrate 82 in which a collimation ball 86 is arranged and adiode 88 arranged at the surface or at a distance of said ball 86 sothat the direction defined by the center of diode 88 and the centre ofball 86 is substantially perpendicular to a diffraction grating 140arranged so as to close opening 100 on the side of the surface ofsubstrate 82 having diffraction gratings 92 and 92′. The light suppliedby diode 88 is collimated by ball 86 so that most of the light reachesgrating 140 with a substantially perpendicular direction. Grating 140has a spatial period and a profile determined so that most of theluminous intensity incident upon grating 140 is diffracted substantiallyin equal parts into the <<+1>> and <<−1>> diffraction orders. The angleof diffraction in the air with respect to the direction perpendicular tograting 140 is for example comprised between 20° and 50°. Thus, most ofthe luminous intensity provided by diode 88 is transmitted in usefulbeams FI and FI′. Grating 140 can be formed in a SiO₂ or Si₃N₄ layer orin a multi-layered structure including in particular a superficialdielectric layer of index n greater than 2.0. Gratings 92 and 92′ areformed at the surface of substrate 82 by deposition of a metal layer 142followed by deposition of a dielectric layer 144, for example SiO₂ orSi₃N₄ Alternatively, the grating can be first etched into substrate 82followed by metal deposition.

In a variant, it is possible to provide a polarisation element betweenball 86 and grating 140. In another variant, it is possible to provide atransparent layer formed in substrate 82 and defining the bottom ofrecess 100. On this transparent layer is deposited a dielectric layer inwhich are formed grating 140 and gratings 92, 92′. It will be noted thatany light source may be provided in this embodiment, fixed to substrate82 or at a distance from the latter. Preferably, the incident light overgrating 140 is substantially collimated. However, even for a divergingsource, grating 140 allows transmission into the <<0>> diffraction orderto be limited and thus the luminous intensity to be concentrated alongdirections defining a non zero angle of incidence on grating 90.

FIGS. 18 and 19 show another embodiment of the invention allowing adisplacement along two orthogonal axes of displacement X and Y to bemeasured. The arrangement along axis X, Y respectively corresponds tothe embodiment described hereinbefore in FIG. 17. A bi-directionalgrating 150 diffracting along directions X and Y is arranged onreflective substrate 112. This bi-directional grating 150 is formed on aset of studs 152 defining grating lines along axes X and Y respectively.It may also be formed by a set of recesses or square hollows, regularlydistributed along axes X and Y. Bi-directional grating 150 shown in FIG.18 is mobile relative to the portion forming the measuring head shown inFIG. 19 and corresponding to the portion associated with the source. Themeasuring head includes on one of its faces arranged facing grating 150,a bi-directional grating 140A having the same function as grating 140along the two directions X and Y. Grating 140A diffracts a light ofnormal incidence essentially into the first diffraction order indirections X and Y. Dotted line 154 represents an opening in themeasuring head while the light source supplying a substantiallycollimated beam is represented by dotted line 156. Grating 140A isformed of studs or square hollows 158 aligned along the two directions Xand Y. The measuring head further includes four gratings 92, 92′, 92Aand 92A′ of period Λ/2 and at least four detectors 98, 98′, 98A and 98A′arranged so as to allow optical paths along the two directions X and Yas shown in the embodiment of FIG. 17 for a unidirectional displacementalong axis X.

It will be noted that, in a less perfected variant, it is possible touse a diverging source, in particular the source 110 shown in FIG. 16,and to omit diffraction grating 140A. It will also be noted that theembodiments shown in FIGS. 1 to 8 can each also be arranged in abi-directional displacement device. In order to do this, the lightsource in particular is arranged so as to emit light along the twodirections X and Y in a direction of propagation which is notperpendicular to the diffraction grating of period Λ similar tobi-directional grating 150 shown in FIG. 18. In the case of a collimatedbeam, in particular a laser beam, this beam will be oriented in anon-perpendicular way with respect to the measuring device grating andwill have a direction, in projection in the plane X-Y, median to axes Xand Y.

Another use according to the invention of the devices corresponding toFIGS. 2, 4, 5, 7, 8, 9, 16, 17, 18 or 19 is the measurement of therelative velocity V along direction X between two gratings, by measuringthe instantaneous frequency f of the modulated signal detected in thedirection of beam FR by at least one detector. The relationship betweenf and V is given by V=Λf/4. It allows a direct measurement of thevelocity without resorting to phase measurement and period counting.

A further embodiment of the invention for velocity measurementcorresponds to FIGS. 2, 4 or 8 whereby grating 48 or 106 is the roughsurface of the moving substrate 50 or 108 whose Fourier component alongcoordinate X corresponding to the spatial frequency of period Λ/2 hasnon-zero amplitude. Substrate 50 or 108 can be a moving band or wire.Among all the beams scattered in all directions at points B1 and B2illuminated by beams 8 and 10, only those diffracted in directions 16and 18 by the spatial frequency corresponding to the spatial period Λ/2will interfere after recombination along beam FR by grating 46 or 104.Two conditions may preferably be fulfilled for a high constructiveinterference to take place along the beam FR. The first condition isthat the rough surface of substrate 50 or 108 is placed at a distancefrom grating 46 where beams 8 and 10 have a non-zero spatial overlap onsaid surface. The second condition is that the length difference AB2-AB1(FIGS. 1 and 2) between beams 10 and 8 is smaller than the coherencelength of source 2. This interference appears as a peak of frequency fin the temporal frequency spectrum of the optical power detected by atleast one detector, f being related to the instantaneous velocity V ofsubstrate 50 or 108 by V=Λf/4. Those familiar with the art will easilylocate f in the frequency spectrum by resorting to spectral analysisinstruments dedicated to Doppler velocimetry. The advantages of thedevice according to the invention for velocity measurement are theminiaturization, the possible small spacing between the readout head,comprising the light source, the detector and the grating of period Λ,and the moving substrate. Another advantage is the possibility of usinga Light Emitting Diode.

A further embodiment of the invention for velocity measurement relatesto the previous embodiment where grating 48 is the surface, exhibiting anon-zero spatial component at period Λ/2, of a substrate 50 moving atvelocity V. The distinct characteristics with respect to the previousembodiment is that the transparent grating 46 of period Λ no longer hasa fixed position relative to the source and to the detector, buttranslates at a constant and known velocity v_(r) along X, v_(r) beinglarger than the maximum which V can have. In one variant, grating 46 isa radial grating made at the surface of a large radius disk rotating ina plane parallel to the displacement direction X and normal to the planeof incidence of beam F1. In a second variant, grating 46 is a closedgrating band rotating on two drums having their rotation axis normal tothe incidence plane, the movement of grating 46 between thesource/detector assembly and the substrate 50 being rectilinear and inthe X direction. Grating 46 is for instance made by embossing in apolymeric foil. The frequency f of the modulated optical power signalmesured by the detector is related to the velocities V and v_(r) throughf=4/Λ(V+v_(r)). This embodiments allows the accurate and fastmeasurement of the velocity V even in case V is close to zero. As aconsequence, this embodiment allows an accurate determination of thelength of a finite displacement L inclusive of its slow beginning and ofits slower end by integrating the velocity V over time t.$L = {{\int_{t_{0}}^{t}{V{\mathbb{d}t}}} = {{\frac{\Lambda}{4}{\int_{t_{0}}^{t}{f{\mathbb{d}t}}}} - {v_{r}\left( {t_{1} - t_{0}} \right)}}}$where t₀ and t₁ are the starting and stop times of the displacement. Thedevice according to the invention can therefore be advantageously usedto measure the length of long strands of wire, bands, ribbons or sheetsof different materials.

FIG. 20 shows an embodiment of a measuring device with a mobile scale160 allowing a maximum measurement range for a given grating length andhaving in addition the advantage that the whole set of the gratings,source, detector(s) and optical paths used for the measurement isentirely contained in a closed case (a tube for example), without themobile grating associated with scale 160 exiting the case, while thedisplacement range of this scale (a metal rod for example) can reach avalue only slightly smaller than the length of the inner cavity 164 ofcase 162, and without the scale 160 supporting the source and thedetector. In order to do this, a light source 166 emits a beam FI alonga direction essentially parallel to direction of displacement X. Rod 160has in its upper portion a plane 168 inclined at an angle greater than45° relative to axis X. This inclined plane 168 defines a mirror forbeam FI, which is reflected in the direction of a fixed grating 170 ofperiod Λ arranged on a wall of cavity 164. Beam FI thus reaches grating170 at an angle of incidence which is not zero according to theinvention. Scale or rod 160 also includes a reflective surface 162defining a grating 174 of period Λ/2. Following grating 174 is arrangedan inclined plane 176 defining a mirror. This inclined plane 176 definesan angle, relative to a direction perpendicular to gratings 170 and 174,equal to the angle defined between inclined plane 168 and direction X.Thus, the resulting beam FR is reflected along a direction parallel toaxis X and is directed towards detector 178.

Those skilled in the art will understand that it is possible to invertthe arrangement of source 166 and detector 178, the optical pathsremaining the same and the light propagating in a reverse direction tothat shown in FIG. 20. In order to assure a stable displacement alongaxis X, two bearings 180 and 182 are provided at the opposite end tothat where the source and the detector are arranged. It will be notedthat any other guide means, in particular a slide can be provided as analternative arrangement.

Other variants using mirrors to deviate and orient incident beam FI andresulting beam FR can be designed by those skilled in the art whileremaining within the scope of the present invention and, in particular,of the embodiment described with reference to FIG. 20.

Finally, it will be noted that the gratings can be formed in variousways by various methods known to those skilled in the art, in particularby a periodic variation in the refractive index at the surface of aplane dielectric layer. Moulding and embossing techniques may also beenvisaged. The profiles of the transverse sections of the diffractiongratings can be optimised for each particular device in order toincrease the efficiency of the displacement measurement according to theprinciple of the invention.

1. A device utilizing light diffraction for measuring translation,rotation or velocity, the device comprising: a light source emitting anincident light beam; at least one light detector for detecting aresultant portion of the incident light beam emitted from the lightsource interference beam; a diffraction grating assembly located on alight path of the incident light beam between the light source and theat least one light detector, the diffraction grating assembly comprisinga fixed first reflective grating assembly and a mobile second reflectivegrating assembly, ; wherein the mobile first grating assembly is mobilealong a given displacement relative to the fixed second gratingassembly; , wherein the fixed reflective grating assembly and the mobilereflective grating assembly diffract first and second grating assembliesare arranged to diffract at least a portion of the incident light beam,the incident light beam reaching the first grating assembly where theincident light beam is partially diffracted along two differentdirections thereby producing interference and the resultant portion ofthe incident light beam detected by the at least one light detectorforming two partial light beams which reach the second grating assembly,and, thereafter, the first grating assembly, thereby forming, afterdiffraction by the first grating assembly, the resultant interferencebeam resulting from interference of the two partial light beams along anoutput direction, and wherein the resultant interference beam isdirected at a resultant angle in a plane perpendicular to lines alongwhich the first grating assembly and the second grating assembly areformed, the resultant angle having a value substantially equal to anangle of incidence in this perpendicular plane of the incident lightbeam multiplied by <<−1>> relative to an axis perpendicular to the firstgrating assembly and the second grating assembly, said at least onedetector being arranged such that light beams interfering along theresultant-angle are measured by the at least one detector fordetermining a relative displacement.
 2. A device according to claim 1,wherein the fixed first grating assembly comprises a first reflectivegrating and the mobile second grating assembly comprises a secondreflective grating.
 3. A device according to claim 1, wherein the fixedfirst grating assembly comprises a first reflective grating and a fourthreflective grating and the mobile second grating assembly comprises asecond reflective grating and a third reflective grating, wherein thefirst grating and the fourth grating are of a first spatial period andare located substantially in a first plane, and the second grating andthe third grating are of a second spatial period and are locatedsubstantially in a second plane, wherein the first plane is displacedform the second plane.
 4. A device according to claim 1, wherein thefixed first grating assembly is mobile relative to the incident lightbeam, and the mobile second grating assembly is fixed relative to theincident light beam and is arranged between the light source and the atleast one light detector.
 5. A device according to claim 4, wherein themobile second grating assembly, the source, and the at least onedetector form an integrated measuring head and the fixed first gratingassembly further comprises a first reflective grating that defines ascale for the device.
 6. A device according to claim 5, wherein thedetector is integrated in a semiconductor substrate bearing the mobilesecond grating assembly.
 7. A device according to claim 5, wherein thelight source is integrated in a semiconductor substrate being the mobilesecond grating assembly.
 8. A device according to claim 1, wherein thefixed first grating assembly has a first spatial period and the mobilesecond grating assembly has a second spatial period that is half thefirst spatial period of the fixed first grating assembly.
 9. A deviceaccording to claim 4, wherein the fixed first grating assembly has aspatial period and the mobile second grating assembly has a secondspatial period that is half the first spatial period of the fixed firstgrating assembly.
 10. A device according to claim 8, wherein theresultant portion of the incident light beam is directed at a resultantangle relative to a plane perpendicular to lines along which the fixedgrating assembly and the mobile grating assembly are formed, theresultant angle having a value substantially equal to an angle ofincidence of the incident light beam multiplied by <<−1>> relative to anaxis perpendicular to the fixed grating assembly and the mobile gratingassembly, such that only light beams interfering along theresultant-angle are measured by the at least one detector fordetermining a relative displacement.
 11. A device according to claim 9,wherein the resultant portion of the incident light beam is directed ata resultant angle relative to a plane perpendicular to lines along whichthe fixed grating assembly and the mobile grating assembly are formed,the resultant angle having a value substantially equal to an angle ofincidence of the incident light beam multiplied by <<−1>> relative to anaxis perpendicular to the fixed grating assembly and the mobile gratingassembly, such that only light beams interfering along the resultantangle are measured by the at least one detector for determining arelative displacement.
 12. A device according to claim 10 1, wherein theincident light beam enters the fixed first grating assembly at the angleof incidence which is not zero in the plane perpendicular to the linesalong which the fixed first grating assembly and the mobile secondgrating assembly are formed, the angle of incidence being sufficient sothat the light source and a detection region of the at least onedetector are spatially separated from each other in projection in theperpendicular plane perpendicular to the lines along which the fixedgrating assembly and the mobile grating assembly are formed .
 13. Adevice according to claim 1, wherein the fixed first grating assemblycomprises a dielectric layer of index n greater than 1.8.
 14. A deviceaccording to claim 1, wherein the mobile second grating assemblycomprises a dielectric layer on top of a reflective substrate.
 15. Adevice according to claim 2, wherein the first grating and the secondgrating are formed of several longitudinal secondary gratings of closebut different frequencies thereby allowing an absolute displacementmeasurement over at least one range of measurement.
 16. A deviceaccording to claim 3, wherein the first grating, the second grating, thethird grating and the fourth grating are formed of several longitudinalsecondary gratings of close but different frequencies thereby allowingan absolute displacement measurement over at least one range ofmeasurement.
 17. A device according to claim 1, further comprising atleast one diffraction grating arranged beside at least one of the fixedfirst grating assembly and the mobile second grating assembly so as todefine at least one reference position for the at least one detector.18. A device according to claim 1 17, further comprising wherein said atleast one diffraction grating having has at least one offset or phasejump incorporated with the lines of the at least one diffraction gratingso as to define at least one reference position for the at least onedetector.
 19. A device according to claim 1, wherein the at least onedetector is arranged for measuring a relative velocity between the fixedfirst grating assembly and the mobile second grating assembly, wherein asole measurement of a frequency of detected luminous intensitymodulation provides the relative velocity.
 20. A device according toclaim 2, wherein at least one of the first grating and the secondgrating has a region with lines offset or phase shifted relative tolines of an other region.
 21. A device according to claim 3, wherein atleast one of the first grating, the second grating, the third gratingand the fourth grating has a region with lines offset or phase shiftedrelative to lines of an other region.
 22. A device according to claim 2,wherein at least one of the first grating and the second grating has aregion formed of at least two secondary gratings having a same periodand a same phase shifted or off set lines, the phase shifted or off setlines being provided so that the resultant portion of the incident lightinterference beam comprises two distinct beams that interfere andproduce alternating luminous intensity signals varying as a function ofrelative position between the fixed first grating assembly and themobile second grating assembly, whereby the alternating luminousintensity signals permits interpolation in an electric period of theluminous intensity signals and allows detection of a relativedisplacement direction between the fixed first grating assembly and themobile second grating assembly.
 23. A device according to claim 3,wherein at least one of the first grating, the second grating, the thirdgrating and the fourth grating has a region formed of at least twosecondary gratings having a same period and a same phase shifted or offset lines, the phase shifted or off set lines being provided so that theresultant portion of the incident light interference beam comprises twodistinct beams that interfere and produce alternating luminous intensitysignals varying as a function of relative position between the fixedfirst grating assembly and the mobile second grating assembly, wherebythe alternating luminous intensity signals permits interpolation in anelectric period of the luminous intensity signals and allows detectionof a relative displacement direction between the fixed first gratingassembly and the mobile second grating assembly.
 24. The deviceaccording to claim 5, wherein the light source comprises anelectroluminescent diode.
 25. The device according to claim 24, furthercomprising an optical collimation element arranged between the lightsource and the first grating.
 26. A device according to claim 2, whereinthe light source emits the incident light beam so that the incidentlight beam comprises a first partial beam incident upon the fixed firstgrating assembly at a positive angle of incidence and a second partialbeam incident upon the fixed first grating assembly at a negative angleof incidence, the fixed first grating assembly and the mobile secondgrating assembly being arranged on either side of two regions ofincidence respectively defined by the first partial beam and the secondpartial beam incident upon the fixed first grating assembly so as toform first to fourth diffracted beams and then to generate at least tworesultant interference between the fourth diffracted beam and a fifthdiffracted beam, thereby producing light beams which are detected oneither side of the two regions by at least two light detectors arrangedon either side of the two regions of incidence.
 27. A device accordingto claim 8, wherein the light source emits the incident light beam sothat the incident light beam comprises a first partial beam incidentupon the fixed first grating assembly at a positive angle of incidenceand a second partial beam incident upon the fixed first grating assemblyat a negative angle of incidence, the fixed first grating assembly andthe mobile second grating assembly being arranged on either side of tworegions of incidence respectively defined by the first partial beam andthe second partial beam incident upon the fixed first grating assemblyso as to form first to fourth diffracted beams and then to generate atleast two resultant interference between the fourth diffracted beam anda fifth diffracted beam, thereby producing light beams which aredetected on either side of the two regions by at least two lightdetectors arranged on either side of the two regions of incidence.
 28. Adevice according to claim 26 40, wherein the source is attached to themobile second grating assembly so that a portion of the mobile secondgrating assembly is situated on either side of the source and offset orphase shifted relative to each other portion so that alternating lightsignals resulting from interference as detected by the at least twodetectors are phase shifted by Λ/2, wherein Λ is a spatial period.
 29. Adevice according to claim 26 40, further comprising a fifth diffractiongrating arranged between the source and the first grating.
 30. A deviceaccording to claim 26 40, wherein the source provides a substantiallycollimated beam propagating along a direction substantiallyperpendicular to the first grating.
 31. A device according to claim 2,wherein at least one of the first grating or the second grating definesa bi-directional diffraction grating having a same spatial period alongtwo orthogonal axes.
 32. A device according to claim 2, furthercomprising at least first and second reflective surfaces, the firstreflective surface arranged to deviate a first beam originating from thesource and propagating substantially along a displacement direction inthe direction of the first of the second reflective grating in order toprovide the incident beam, and the second reflective surface arranged toreflect the interfering light along an the output directionsubstantially in a direction parallel to the displacement directionbefore being detected by the at least one detector.
 33. A deviceaccording to claim 32 43, wherein the source and the at least onedetector are attached to the fixed first grating assembly and the firstand second reflective surfaces are formed on a rod supporting the mobilesecond grating assembly.
 34. A device according to claim 6, wherein thelight source is integrated in a semiconductor substrate bearing thesecond grating assembly.
 35. A device utilizing light diffraction formeasuring translation, rotation or velocity, the device comprising: alight source emitting an incident light beam; at least one lightdetector for detecting a resultant interference beam; a diffractiongrating assembly located on a light path of the incident light beambetween the light source and the at least one light detector, thediffraction grating assembly comprising a first reflective gratingassembly having a first reflective grating and a second reflectivegrating assembly having a second reflective grating; wherein the gratingassembly is mobile along a given displacement relative to the secondgrating assembly, wherein the first and second grating assemblies arearranged to diffract at least a portion of the incident light beam, theincident light beam reaching the first grating assembly where theincident light beam is partially diffracted along two differentdirections thereby forming two partial light beams which reach thesecond grating assembly, and, thereafter, the first grating assembly,thereby forming, after diffraction by the first grating assembly, theresultant interference beam resulting from interference of the twopartial light beams along an output direction, wherein the first gratingand the second grating are formed of several longitudinal secondarygratings of close but different frequencies thereby allowing an absolutedisplacement measurement over at least one range of measurement.
 36. Adevice utilizing light diffraction for measuring translation, rotationor velocity, the device comprising: a light source emitting an incidentlight beam; at least one light detector for detecting a resultantinterference beam; a diffraction grating assembly located on a lightpath of the incident light beam between the light source and the atleast one light detector, the diffraction grating assembly comprising afirst reflective grating assembly and a second reflective gratingassembly; and at least one diffraction grating arranged beside at leastone of the first grating assembly and the second grating assembly so asto define at least one reference position for the at least one detector;wherein the first grating assembly is mobile along a given displacementrelative to the second grating assembly, wherein the first and secondgrating assemblies are arranged to diffract at least a portion of theincident light beam, the incident light beam reaching the first gratingassembly where the incident light beam is partially diffracted along twodifferent directions thereby forming two partial light beams which reachthe second grating assembly, and, thereafter, the first gratingassembly, thereby forming, after diffraction by the first gratingassembly, the resultant interference beam resulting from interference ofthe two partial light beams along an output direction.
 37. A deviceutilizing light diffraction for measuring translation, rotation orvelocity, the device comprising: a light source emitting an incidentlight beam; at least one light detector for detecting a resultantinterference beam; a diffraction grating assembly located on a lightpath of the incident light beam between the light source and the atleast one light detector, the diffraction grating assembly comprising afirst reflective grating assembly and a second reflective gratingassembly; and at least one diffraction grating arranged beside at leastone of the first grating assembly and the second grating assembly, saidat least one diffraction grating having at least one offset or phasejump, incorporated with the lines of the at least one diffractiongrating so as to define at least one reference position for the at leastone detector; wherein the first grating assembly is mobile along a givendisplacement relative to the second grating assembly, wherein the firstand second grating assemblies are arranged to diffract at least aportion of the incident light beam, the incident light beam reaching thefirst grating assembly where the incident light beam is partiallydiffracted along two different directions thereby forming two partiallight beams which reach the second grating assembly, and, thereafter,the first grating assembly, thereby forming, after diffraction by thefirst grating assembly, the resultant interference beam resulting frominterference of the two partial light beams along an output direction.38. A device utilizing light diffraction for measuring translation,rotation or velocity, the device comprising: a light source emitting anincident light beam; at least one light detector for detecting aresultant interference beam; a diffraction grating assembly located on alight path of the incident light beam between the light source and theat least one light detector, the diffraction grating assembly comprisinga first reflective grating assembly having a first reflective gratingand a second reflective grating assembly having a second reflectivegrating; wherein the first grating assembly is mobile along a givendisplacement relative to the second grating assembly, wherein the firstand second grating assemblies are arranged to diffract at least aportion of the incident light beam, the incident light beam reaching thefirst grating assembly where the incident light beam is partiallydiffracted along two different directions thereby forming two partiallight beams which reach the second grating assembly, and, thereafter,the first grating assembly, thereby forming, after diffraction by thefirst grating assembly, the resultant interference beam resulting frominterference of the two partial light beams along an output direction,wherein the light source emits the incident light beam so the incidentlight beam comprises a first partial beam incident upon the fixedgrating assembly at a positive angle of incidence and a second partialbeam incident upon the fixed grating assembly at a negative angle ofincidence, the first grating assembly and the second grating assemblybeing arranged on either side of two regions of incidence respectivelydefined by the first partial beam and the second partial beam incidentupon the first grating assembly so as to form first to fourth diffractedbeams and then to generate at least two resultant interference beamswhich are detected on either side of the two regions by at least twolight detectors arranged on either side of the two regions of incidence.39. A device according to claim 38, wherein the first grating assemblyhas a first spatial period and the second grating assembly has a secondspatial period that is half the first spatial period of the firstgrating assembly.
 40. A device utilizing light diffraction for measuringtranslation, rotation or velocity, the device comprising: a light sourceemitting an incident light beam; at least one light detector fordetecting a resultant interference beam; a diffraction grating assemblylocated on a light path of the incident light beam between the lightsource and the at least one light detector, the diffraction gratingassembly comprising a first reflective grating assembly having a firstreflective grating and a second reflective grating assembly having asecond reflective grating, wherein at least one of the first grating orthe second grating defines a bi-directional diffraction grating having asame spatial period along two orthogonal axes; wherein the first gratingassembly is mobile along a given displacement relative to the secondgrating assembly, wherein the first and second grating assemblies arearranged to diffract at least a portion of the incident light beam, theincident light beam reaching the first grating assembly where theincident light beam is partially diffracted along two differentdirections thereby forming two partial light beams which reach thesecond grating assembly, and, thereafter, the first grating assembly,thereby forming, after diffraction by the first grating assembly, theresultant interference beam resulting from interference of the twopartial light beams along an output direction.
 41. A device utilizinglight diffraction for measuring translation, rotation or velocity, thedevice comprising: a light source emitting an incident light beam; atleast one light detector for detecting a resultant interference beam; adiffraction grating assembly located on a light path of the incidentlight beam between the light source and the at least one light detector,the diffraction grating assembly comprising a first reflective gratingassembly having a first reflective grating and a second reflectivegrating assembly having a second reflective grating; and at least firstand second reflective surfaces, the first reflective surface arranged todeviate a first beam originating from the source and propagatingsubstantially along a displacement direction of the second reflectivegrating in order to provide the incident beam, and the second reflectivesurface arranged to reflect interfering light along the output directionsubstantially in a direction parallel to the displacement directionbefore being detected by the at least one detector; wherein the firstgrating assembly is mobile along a given displacement relative to thesecond grating assembly, wherein the first and second grating assembliesare arranged to diffract at least a portion of the incident light beam,the incident light beam reaching the first grating assembly where theincident light beam is partially diffracted along two differentdirections thereby forming two partial light beams which reach thesecond grating assembly, and, thereafter, the first grating assembly,thereby forming, after diffraction by the first grating assembly, theresultant interference beam resulting from interference of the twopartial light beams along an output direction.